1. Field of the Invention
The present invention relates to calibration of a printer and, more particularly, to calibration of a printer using a tone reproduction curve, the calibration requiring no measuring equipment.
2. Description of the Related Art
Electronic printers are generally capable of producing gray tones, as well as standard black and white tones in order to produce gray tones. The printers accept gray level specifications as input and produce corresponding gray areas on a printed page. This production of the corresponding gray areas is often performed by a halftoning operation wherein a fine pattern of black and white dots is printed, the dots appearing as varying gray tones in accordance with the number of black and white dots used when viewed from a distance. The allowed gray level specifications generally vary over some finite range such as 0 to 1 or 0 to 255. The colors at the extremes (i.e., 0 and 1 or 0 and 255) are white and black, numbers between the extremes yielding intermediate gray tones. However, while the gray level specifications may vary linearly, a linear change in gray levels typically does not result. Instead, there may be a particular threshold before lighter toned grays become visible. Similarly, darker toned grays may prematurely appear black.
In order to accurately produce a desired gray level, one must know the correspondence between the gray level specification of the printer and the actual gray level produced by the printer. A tone reproduction curve (TRC) provides an illustration of the corresponding relationship. If the TRC is known, one can compensate for the nonlinearities of the printer.
The TRC, which is dependent upon the stability of the printing device, may change frequently, or the halftone used to produce gray levels may be altered. Thus, to accurately assess the corresponding relationship, the TRC must be measured frequently.
In order to determine the TRC, a sample set of gray level specifications can be printed and the gray levels actually produced by the device can be measured by a device such as a densitometer. The measured points can then be fitted to a curve, this curve being the TRC.
This method of determining the TRC, however, requires special equipment such as the densitometer to measure the gray level. Further, a skilled operator is desired to accurately assess the results of use of the densitometer. The special equipment and skilled operator are not always available for use with every printer.
Accordingly, it would be advantageous to enable determination of the TRC without requiring the use of special equipment and/or a skilled operator.
U.S. Pat. No. 4,941,057 to Lehmbeck et al. discloses a digital image proofing process. The processing arrangement allows a number of image processing techniques to be tested on a single sample sheet at a single time. A sample segment of the image may be selected for testing, and successively repeated in a sample output for the number of image processing techniques to be tested. The proof pages are printed on the printer, with the output of the proof page demonstrating the system operation with the desired image processing technique. The reference provides no disclosure of calibration of a printer.
U.S. Pat. No. 3,489,849 to Hedger discloses a facsimile test pattern generator. The method consists of a test set for testing the operation of a system adapted to reproduce and to record a facsimile in response to receipt signals. The method of the reference also provides no disclosure of a calibration method for a printer.